Not recognizing calc result as number, nor recognizing @property

Below is an excerpt of css being implemented in a WordPress mechanism to generate css-only Low Quality Image Placeholders (https://github.com/WordPress/performance/pull/2550/changes), inspired by this article https://leanrada.com/notes/css-only-lqip/

@property --lqip {
	syntax: "<number>";
	inherits: true;
	initial-value: 0;
}

[style*="--lqip:"] {
	--lqip-ca: mod(round(down, calc((var(--lqip) + 524288) / 262144)), 4);
	--lqip-cb: mod(round(down, calc((var(--lqip) + 524288) / 65536)), 4);
	--lqip-cc: mod(round(down, calc((var(--lqip) + 524288) / 16384)), 4);
	--lqip-cd: mod(round(down, calc((var(--lqip) + 524288) / 4096)), 4);
	--lqip-ce: mod(round(down, calc((var(--lqip) + 524288) / 1024)), 4);
	--lqip-cf: mod(round(down, calc((var(--lqip) + 524288) / 256)), 4);
	--lqip-ll: mod(round(down, calc((var(--lqip) + 524288) / 64)), 4);
	--lqip-aaa: mod(round(down, calc((var(--lqip) + 524288) / 8)), 8);
}

W3C CSS Validator shows these errors

1		Unrecognized at-rule @property
8	[style*="--lqip:"]	The types are incompatible ), 4)
9	[style*="--lqip:"]	The types are incompatible ), 4)
10	[style*="--lqip:"]	The types are incompatible ), 4)
11	[style*="--lqip:"]	The types are incompatible ), 4)
12	[style*="--lqip:"]	The types are incompatible ), 4)
13	[style*="--lqip:"]	The types are incompatible ), 4)
14	[style*="--lqip:"]	The types are incompatible ), 4)
15	[style*="--lqip:"]	The types are incompatible ), 8)
15	[style*="--lqip:"]	Parse Error

As noted in a code review comment there, this is what Claude Opus 5 said

Both tools are wrong, and about the same thing: the two‑argument round(down, A) form where the rounding interval B is omitted.

What the spec says

css-values-4 §10.3:

The round(<rounding-strategy>?, A, B?) function … If the type of A matches <number>, then B may be omitted, and defaults to 1. … In all other cases, omitting B is invalid.

--lqip is registered as syntax: "<number>" (lqip.css:12-16), so A is a <number> and omitting B is legal. The code is correct.

Why the error points at mod() instead of round()

Neither tool can establish a type for the round() result, so it falls through to mod()'s "both arguments must be the same type" check against the literal 4/8. That's also why lqip.css:27 — mod(calc(var(--lqip) + 524288), 8), no round() — is clean in both tools while lines 19‑26 are not.

The two tools fail for different reasons

I probed both with variants:

Expression	PhpStorm	W3C
round(down, 5)	ok	error
round(down, var(--lqip))	ok	—
round(down, calc(var(--lqip) + 1))	error	error
round(down, …, 1) (explicit interval)	ok	ok

• PhpStorm does implement the number default — it even reads your @property to type a bare var(--lqip). It just loses the type when the argument is a calc() containing a var(). Dropping the redundant calc() wrapper (round(down, (var(--lqip) + 524288) / 262144)) doesn't help.
• W3C/Jigsaw doesn't implement the omitted-B rule at all — even round(down, 5) reports "The types are incompatible."

Browsers disagree with both

In Chrome 152, with --lqip: 192900, the omitted-interval and explicit-interval forms both compute --lqip-ca: 2, and CSS.supports('opacity', 'round(down, 0.5)') is true.

If you want the warnings gone

Adding the explicit , 1 to the eight round() calls is a semantic no-op that satisfies both tools. Verified on a copy: W3C goes from 9 errors to 1, PhpStorm to 0.

--lqip-ca: mod(round(down, calc((var(--lqip) + 524288) / 262144), 1), 4);

The remaining W3C error is Unrecognized at-rule "@property" at lqip.css:12 — a separate Jigsaw gap (it predates css-properties-values-api), and nothing you can or should work around.

So, there's two issues:

1. not recognizing the calc as number
2. Not recognizing @property

This resolves the calc/round error, but shouldn't be necessary (and isn't, in actual browsers)

	--lqip-ca: mod(round(down, calc((var(--lqip) + 524288) / 262144), 1), 4);
	--lqip-cb: mod(round(down, calc((var(--lqip) + 524288) / 65536), 1), 4);
	--lqip-cc: mod(round(down, calc((var(--lqip) + 524288) / 16384), 1), 4);
	--lqip-cd: mod(round(down, calc((var(--lqip) + 524288) / 4096), 1), 4);
	--lqip-ce: mod(round(down, calc((var(--lqip) + 524288) / 1024), 1), 4);
	--lqip-cf: mod(round(down, calc((var(--lqip) + 524288) / 256), 1), 4);
	--lqip-ll: mod(round(down, calc((var(--lqip) + 524288) / 64), 1), 4);
	--lqip-aaa: mod(round(down, calc((var(--lqip) + 524288) / 8), 1), 8);

Reproduce the supplied CSS in the validator, then trace how it parses @property and types calc(), round(), and mod(). Compare the results with the CSS Values specification and browser behavior. Done means valid @property declarations and number expressions validate without requiring an explicit rounding interval.